The present invention relates to a numerical control apparatus which operates to detect follow-up error, and which operates to precisely and synchronously a rotation control angle of a main shaft (spindle), of an NC (numerical control) lathe for example, and a moving position of a feed shaft.
FIG. 1 is a block diagram showing an example of the conventional NC apparatus for synchronously controlling a rotational angle of a main shaft, of a conventional NC lathe and the like and a position of a feed shaft. As shown, the conventional NC apparatus has a main shaft pulse generator 8 for detecting a rotational angle of a main shaft 7, a pulse counter 9 for counting A and B phase pulses, and a marker pulse start signal, generated by the main shaft pulse generator 8, a latch circuit 12 for reading, through a positional detector 10 and a counter 11, the present position of a feed shaft driven by a servo motor 6 in response to a present position reading signal sent from the pulse counter 9, and a positional data table for command 1 positional command values storing and for generating integrally off-line a positional command value of the feed shaft corresponding to the pulses denoting a single rotation of the main shaft which are generated by the main shaft pulse generator 8. Furthermore, the conventional NC apparatus comprises a substracter 2 for determining a positional deviation value, in which a table address signal received from the pulse counter 9 is utilized as a control timing, by subtracting the present position of the latch circuit 12 from the positional command value read from the positional command data table 1. Also provided are a D/A converter for controlling the servomotor 6 according to the positional deviation value outputted by the subtracter 2 and synchronizing the position of the feed shaft with the rotational angle of the main shaft 7, an analog-gain amplifier 4 and a servo-amplifier 5.
In the operation of such a conventional NC apparatus, synchronous control is effected every time the main shaft pulse generator 8 outputs the A and B phase pulses. When the next marker pulse is outputted, the control returns to the first positional command value and the same control operation is repeated.
Since the above conventional NC apparatus outputs a positional command value which has been previously stored in response to and corresponding to the pulse signals generated by the main shaft pulse generator 8, it is not necessary to have a time in which a function generation and process is carried out. As a result, response is possible even when the rotational speed of the main shaft is high, thereby enabling a high speed cutting operation. However, since the function generation is not carried out on a real time basis, the follow-up error of the feed shaft disadvantageously becomes accordingly large. Consequently, in order to achieve precision machining it is necessary, to change the operating conditions (for example, by changing the command values) until the accuracy of the workpiece becomes a predetermined value and to then repeat the cutting or machining operation and measurement (for example, the workpiece is removed from the NC lathe after it is cut and then an exclusive-use measurement device is used in a temperature-controlled room to measure the workpiece), thereby requiring the expenditure of much time.